Extrusion mandrel



Nov. 24, 1970 R. 1.. SIMCDNTON 3,541,831

EXTRUSION MANDREL Filed April 7, 1967 Z'Sheets-Sheet 1 l 5V .llHHI /4 mTiqE.

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INVENTOR.

RAYMOND L. SIMONTON BY 7111RPM R. L. SIMONTON EXTRUSION MANDREL Nov. 24,1970 2 Sheets-Sheet 2 Filed April 7, 1967 INVENTOR.

RAYMON D L. SIMONTON M4,. PM

United States Patent Office 3,541,831 Patented Nov. 24, 1970 3,541,831EXTRUSION MANDREL Raymond L. Simonton, Huntington, W. Va., assignor toThe International Nickel Company, Inc., New York,

N.Y., a corporation of Delaware Filed Apr. 7, 1967, Ser. No. 629,162Int. Cl. B211.- 25/06 U.S. Cl. 72266 15 Claims ABSTRACT OF THEDISCLOSURE Compound mandrel for use in hot extrusion of hollow metalproducts has a heat-resistant casing disposed around a tension bar ofmaraging steel.

The present invention relates to metal working apparatus and, moreparticularly, to apparatus for extruding hollow metal articles.

Heretofore, in the metallurgical art, methods and apparatus have beendevelopedfor forming hollow metal articles such as seamless tubes byextruding a heated metal billet over a mandrel and through a die. Forinstance, hot extrusion of tubular products made of high strength,diflicultly formable, high melting point metals such as carbon steel,stainless steel and other difficult to form metals is described in TheMaking, Shaping and Treating of Steel, 8th edition, US. Steel, 1964.

During hot extrusion of tubular products, the mandrel is subjected tosevere heat and to a complex combination of forces including transversecompressive forces from the billet which is being compressed within theextrusion chamber, longitudinal tensile forces that are exerted throughfriction of the extruded metal passing over the mandrel and alsotransverse bending forces. Although theoretical considerations mightindicate that the forces on a mandrel would be exerted symmetricallywith respect to the longitudinal axis of the mandrel, it has been foundin practice that mandrels, especially mandrels of relatively small crosssection, are sometimes bent or broken in bending by the complex forcesof extrusion. Additionally, thermal shock from contact with hot billetsand from cooling between extrusions exerts severe stresses in hotextrusion mandrels and sometimes results in unsatisfactory short life ofhot extrusion mandrels, particularly where the mass of the mandrel ishigh such as in mandrels of -inch diameter or larger. Materials withoutgood thermal shock resistance are not wholly satisfactory for hotextrusion mandrels even if characterized by high strength and otherdesirable properties. For instance, breakage due to thermal shock hasresulted in unsatisfactorily short life of mandrels made of tool steelhaving a nominal composition of 0.35% carbon, 5% chromium, 1% vanadiumand 1.5% molybdenum. The poor thermal shock properties which generallycharacterize cemented carbide materials would be expected to renderthese materials unsatisfactory for hot extrusion mandrels. Extrusionmandrels are also subjected at times to heavy mechanical shocks whichcause breakage if the toughness of the mandrel is inadequate. Furtherditficulties with mandrels in the prior art, e.g., 5% chromium toolsteel mandrels, include heat checking, cracking, wear, scoring andtearing of the exterior surfaces.

In view of the stresses and strains and the heat to which extrusionmandrels are subjected, it is not surprising to find that the mandrelsfail undesirably frequently. The effects of the complex forces and theheat, shock, friction, etc., to which hot extrusion mandrels aresubjected are so various and interrelated as to make it impractical toprecisely predict all the requisite characteristics for satisfactoryextrusion mandrel structures and materials. Accordingly, the art ofextrusion mandrel design is largely empirical. Due to the high cost ofmetal and machining for replacing extrusion mandrels, the failurethereof contributes disadvantageously to increasing the cost ofproducing extruded hollow metal articles. Additionally, mandrel failureresults in costly spoilage of extrusion billet material and loss ofproduction time. For instance, when the head portion is pulled from amandrel during extruding a tube, usually the head becomes lodged in thetube and the remainder of the billet is extruded as a solid bar insteadof as the desired tube. Scoring and other surface deterioration of themandrel detrimentally results in interior surface defects in theextruded products, due to lack of a smooth surface on the mandrel.

In attempting to overcome mandrel failure problems, the art has providedinternal water-cooling means in mandrels with the object of maintainingthe strength of the metal when surrounded by a heated billet. However,water-cooled mandrels in the prior art suffer from further disadvantagesand have not overcome all problems of mandrel failure, one disadvantagebeing that watercooling passages decrease the amount of metal availablefor strength. A further disadvantage of water cooling is that in eventof rupture of the mandrel, water can flow into the interior of the hotbillet and the resulting steam pressure confined in the extrusionchamber can do great damage, such as by violently exploding thepartially extruded tube from the press. Accordingly, a high margin ofstrength is needed for safety in water-cooled hot-extrusion mandrels.Other attempts to provide improved mandrels for hot extrusion includeapplication of hard facing coatings, e.g., tungsten-carbide coatings,high-carbon cobalt alloy coatings and high-carbon steel coatings, buthard facing coatings have suffered from cracking difficulties.

Although many attempts were made to overcome the foregoing difficultiesand other difficulties, none, as far as I am aware, was entirelysuccessful when carried into practice commercially on an industrialscale.

It has now been discovered that a new structure having a new combinationof characteristics provides an extrusion mandrel having improvedresistance to fracture or rupture and improved wear resistance, wherebyincreased service life is obtained.

It is an object of the present invention to provide a new extrusionmandrel having improved resistance to failure by deformation orfracture, including resistance to tensile failure, and also havingimproved resistance to wear, tear and scoring of the outer surface.

A further object of the invention is to provide a new mandrel for use inextruding hollow articles made of metals characterized by highresistance to deformation, heat resistance and/ or high melting points.

Other objects and advantages will become apparent from the followingdescription taken in conjunction with the accompanying drawings inwhich:

FIGS. 1, 2, 3 and 4 illustrate, in longitudinal cross section, fourembodiments of the mandrel provided in accordance with the invention;and

FIG. 4a illustrates a transverse cross section of the mandrel of FIG. 4.

FIG. 5 shows a side view of the mandrel of FIG. 4 attached to a mandrelholder of an extrusion press with the mandrel holder shown inlongitudinal partial cross section.

FIG. 6 illustrates, in partial cross section, cooperation of anextrusion die and metal being extruded into tubular form with themandrel of FIG. 4.

Generally speaking, the present invention is directed to a new extrusionmandrel comprising a high strength, ductile, tension bar composed ofmaraging steel having a heat treated, aged, internal structurecharacterized by a yield strength of at least about 200,000 pounds persquare inch (p.s.i.) and a tensile elongation of at least 7%,advantageously at least about 10%, at room temperature, a full lengthcasing composed of a ductile heat resistant alloy slidably disposedaround the tension bar and a retainer at the forward end of the tensionbar. The retainer is engageable with the front end, i.e., the forwardportion near or at the forward extremity, of the casing and is adaptedto limit forward motion of the casing, thereby restraining the casingfrom being dragged off the tension bar during extrusion. At the rearwardend of the tension bar a threaded stud or other attachment means isprovided for mounting the mandrel in an extrusion press. It is to beparticularly noted that the mandrel components are so arranged thatforward longitudinal movement of the casing is mechanically restrainedonly by the retainer and only at the front end of the casing. Theductile heat resistant casing is characterized by ductility that avoidsfracture in event of moderate bending such as can occur duringextrusion, by metallurgical stability and also by good retention ofhardness and strength when heated to elevated temperatures. Additionalcharacteristics of the casing include thermal shock resistance and wearand abrasion resistance at high temperatures.

During use, when a billet is being extruded over the mandrel, thetension bar sustains essentially all the tensile load exerted byfriction of the forward moving billet metal and, inasmuch as the casingis slidably disposed on the tension bar and is restrained by theretainer only at the front end of the casing, the casing is underlongitudinal compression and is not subjected to tensile loading. It isto be understood herein that reference to compression in a mandrelcomponent refers to compression in the component when taken as a whole.Thus, where the casing as a whole is in compression it is referred to asbeing in compression and not in tension even though small portions ofthe casing may be subjected to some highly localized tensile stress, forinstance, tensile stress such as may be exerted on the outer skin of thecasing by friction where forward-moving extrusion metal contacts thecasing. Of course, during extrusion the casing is also subjected tocomplex stresses including transverse compression in addition tolongitudinal compression. Transverse compressive forces on the casingare transmitted to the tension bar, which resists transverse compressionof the casing and thereby serves as a back-up for the casing. Thus, thetension bar is sometimes simultaneously in transverse compression andlongitudinal tension. It is further pointed out that the full lengthcasing extends over essentially the entire length of the tension bar,thereby preventing wear, scoring and other surface damage to the tensionbar and also protecting the tension bar against heat.

Maraging steels referred to herein are iron-base alloys, i.e., alloyswherein the percentage of iron is greater than the percentage of anyother element present in the alloy, and are generally characterized bylow carbon contents not greater than about 0.15% carbon, advantageouslynot greater than 0.05% carbon, more advantageously not greater than0.03% carbon. In addition to iron, maraging steels generally containabout 10% to about 30% nickel, up to about 30% cobalt, up to about 10%molybdenum and up to about 8% chromium and also contain auxiliaryhardening elements such as titanium, aluminum and/or columbium. Suchsteels are described in US. Pats. Nos. 3,093,518 and 3,093,519. Incommercial practice, tensile strength levels, e.g., 250,000 p.s.i. yieldstrength, are commonly employed in designating maraging steels. Yieldstrengths referred to herein are determined by the 0.2% offset method;tensile elongations are with one-inch gage lengths. Advantages of thenew mandrel tension bar having, in accordance with the invention, amaraged microstructure include high tensile strength, ductility,toughness and freedom from detrimental deformation during heattreatment. Ductility and toughness, including high notch tensilestrength, are especially required for mandrels wherein thetension-supporting member is of low cross-sectional area, such as influid cooled mandrels and in mandrels of small diameter, e.g., 2% inchesor less diameter, in order to avoid cracking or fracture due to bending.

In carrying the invention into practice, it is highly advantageous, inorder to provide a mandrel having especially high resistance to tensilefailure, that the mandrel comprise a maraging steel tension barcharacterized in the heat treated (maraged) condition by high yieldstrength of at least about 250,000 p.s.i. at room temperature.Advantageous high strength embodiments of the tension bar includetension bars made of maraging steel containing about 17% to about 19%nickel, about 7% to about 9.5% cobalt, about 3% to about 5.2 molybdenum,about 0.15% to about 0.8% titanium, about 0.05 to about 0.15% aluminum,up to about 0.03% carbon with the balance essentially iron and heattreated to have a yield strength on the order of about 250,000 p.s.i. orhigher and a tensile elongation of at least about 10%, e.g., 12%, atroom temperature, e.g., heat treated by heating for about 1 hour atabout 1500 F. followed by air cooling and reheating for about 3 hours atabout 900 F and then air cooling. Toughness is needed in the tension barand use therein of brittle materials, or materials which are susceptibleto embrittlement upon exposure to temperatures encountered by mandrelsduring extrusion, is avoided. In order to obtain full benefits of themaraging steel tension bar in accordance with the invention it isadvantageous, especially in mandrels for hot extrusion of alloys such asnickel-base alloys, ironbase alloys or cobalt-base alloys, that suchmandrels also have fluid coolant means for maintaining the temperatureof the tension bar at less than 900 F., e.g., 700 F. or lower, therebymaintaining in use the high yield strength of the bar.

The ductile heat resistant alloy casing provided by the invention iscomposed of a metallurgically stable alloy containing 'at least 50%nickel, cobalt and/or iron, e.g., a metallurgically stable alloycontaining up to about 21% chromium, advantageously 12% to 21% chromium,up to about 3.5% aluminum, up to about 3% titanium, up to about 6%columbium and/ or tantalum, up to about 10% molybdenum, up to about 1%silicon, up to about 2% manganese, up to about 0.2% carbon and balanceessentially metal from the group consisting of nickel, cobalt and iron.Advantageously, the casing is characterized at temperatures from roomtemperature up to at least about 1000 F. by ductility equal to at least10% tensile elongation and by ultimate tensile strength of at leastabout 120,000 p.s.i. The casing ofthe new mandrel is metallurgicallystable in the sense that the alloy matrix of the casing does not readilyundergo any major phase change when subjected to heating and coolingunder hot extrusion conditions. For instance, where the casing is aniron-base alloy the casing matrix is normally austenitic at room andelevated temperatures; casings made of alloys which undergotransformations from austenite to martensite, or vice versa, 'whenheated and/or cooled under hot extrusion conditions are not inaccordance with the present invention. Advantageous heat resistantalloys for the casing include ductile nickelchromium alloys containingabout 12% to about 21% chromium, up to about 28% iron, up to about 34%cobalt, up to about 3.5% aluminum, up to about 3% titanium, up to about6% columbium or columbium plus tantalum, up to about 10% molybdenum, upto about 1% silicon, up to about 2% manganese and up to about 0.2%carbon with balance substantially nickel in an amount not less thanabout 45% of the alloy. All alloy composi- TABLE I Percent AlloyNo Cr FeCo Al Ti Cb Mo 0 Ni 1 Up to about 20% of the percentages shown forcolumbium may be li zi l f Balance, which can include small amounts ofdeoxidizers, malleabilizers and nondetrimental impurities.

The slidable relationship of the casing to the tension bar is asufficiently close fit, e.g., 0.005-inch clearance, to enable the bar toback-up the casing during extrusion and yet permit the casing to movefreely enough to avoid being stressed longitudinally in tension byfrictional forces during extrusion or by thermal expansion forces whichmay arise from differences in temperature or in thermal expansioncoeflicients of the casing and the tension bar. Close slidable fitswhich permit the casing to be moved on to the bar by hand or by an arborpress are satisfactory. Even though the casing can be press fit to thetension bar and during use may become even more tightly fit (but notwelded) by extrusion, such fits are still slidable in relationship tothe very high forces existent during extrusion or during heating andcooling of the mandrel before and after extrusion. In addition, theunbonded fit promotes flexural advantages for avoiding fracture inbending and permits removal of the casing for replacement when overlyworn or when use of another size casing is required. Incontradistinction to the slidable disposition of the casing on thetension bar in the mandrel of the invention, mandrels having coatings,casings, collars, etc., which are joined by bonding, e.g., welded,brazed, diffusion bonded, etc., and/or which are otherwise rigidlyjoined to a core, whether continuously joined or joined at front andrear portions of the casing, do not have all of the advantages of, andare not in accordance with, the present invention.

The retainer can be integral with the tension bar or can be replaceablyfastened to the front end of the bar, e.g., held by a threaded stud atthe front end of the bar. Where the retainer is not integral with thebar, the retainer and the fastening means therefor are advantageouslymade of high strength maraging steel inasmuch as the retainer issubjected to high forces tending to pull the retainer forwardly from thebar during extrusion.

A new mandrel which, in accordance with the invention, is particularlyadvantageous for hot extrusion of heat resistant nickel-chromium alloyscomprises a casing made of a precipitate-hardened alloy containing about50% to about 55% nickel, about to about 21% chromium, 2% to 4%molybdenum, 4.75% to about 5.5% columbium, 0.65% to about 1.15%titanium, 0.2% to about 0.8% aluminum, up to about 0.1% carbon,advantageously 0.03% to 0.1% carbon, up to about 0.5% silicon, up toabout 0.5% manganese with balance substantially iron, a tension barcomposed of nickel-cobaltmolybdenum maraging steel in the conditioncharacterized at room temperature by yield strength of at least about250,000 p.s.i. and tensile elongation of at least about 10% and alsocomprising fluid coolant means for cooling the tension bar.

Turning now to the drawing, it is to be noted that four specificembodiments of the new mandrel of the invention are illustrated therein.Each of the four illustrated mandrels is of uniform, circular,transverse cross section at the exterior surface and thus is adapted forextruding hollow cylindrical tubes having substantially uniform,circular, cross-sectioned interior passages. The components of themandrels illustrated in FIGS. 1, 2 and 3 are all circular in transversecross section.

FIG. 1 shows mandrel 10 having casing 11 slidably disposed aroundelongated, longitudinally extending tension bar 12 and abutting againstretainer 13. Casing 11 extends essentially the entire length of tensionbar 12. Threaded stud 14 is integral with bar 12 and is mated withthreads 15 in the retainer. Extending from the rear end of the tensionbar, threaded stud 16, which is integral with the bar, is adapted forattaching the mandrel to a mandrel holder in an extrusion press. (It isto be noted that, as referred to herein, a casing extends overessentially the entire length of a tension bar when the casing extendsup to about the ends of the bar even though it does not extend over astud or other connection means on the end of the bar.)

In FIG. 2, mandrel 20 comprises casing 21 and tension bar 22. Retainer2,3 is integral with the tension bar. Casing 21 is slidably disposedover essentially the entire length of the tension bar and abuts againstretaining shoulder 24 of retainer 23 at the forward end of the bar.Threaded stud 25 is integral with the rear of the bar and is adapted forattachment to a mandrel holder.

Mandrel 30, illustrated in FIG. 3, has casing 31 slidably disposed overthe length of tension bar 32 and abutting against retainer 33. Mandrel30 is adapted for piercing in addition to extruding and has detachablepiercing point 34 fitted on stub 35. Threaded stud 36 provides means forattaching the mandrel to an extrusion press.

In FIG. 4, mandrel 40 has casing 41 slidably disposed over the entirelength of grooved tension bar 42 and abutting against retainer 43. Theretainer is mechanically fastened to the forward end of the bar bythreaded stud 44 which is integral with the tension bar. The bar hascentral coolant passage 45 with inlet coolant pipe 46 extending into thecentral passage. Back flow of coolant in the passage is prevented bycoolant seal 47. Transverse coolant ports 48 near the forward end of thebar enable coolant to flow from the central passage to coolant returngrooves 49 and thence to transverse exit port 50. Grooves 49 are open tothe casing, e.g., as illustrated in FIG. 4a, and thus provide for directcontact of the coolant with the casing. Coolant passing through port 50can then fiow rearward through the central passage and along the outsideof the inlet pipe so as to exit at the rear of the mandrel.

FIG. 4a shows the transverse cross section of mandrel 40 taken at line4a4a of FIG. 4.

Mandrel 40 is shown, in FIG. 5, attached to mandrel holder 51 of anextrusion press. Threaded mounting stud 52, which is integral with thetension bar of the mandrel, is mated in thread 53 in the mandrel holder.Coolant passage 54 in the mandrel holder communicates with the centralpassage in the tension bar and coolant inlet pipe 46 extends rearwardthrough passage 54 in the mandrel holder. Both the coolant inlet pipeand passage 54 communicate with coolant supply and coolant exhaustmeans, respectively, which are not shown and fonm no part of theinvention.

FIG. 6 illustrates mandrel 40 and extrusion die 61 in extrusion presschamber 62 during extrusion of metal 63. It is understood that duringnormal operation the extrusion die is stationary and the mandrel moveswith the mandrel holder of the extrusion press while the metal isextruded forwardly (to the right in FIG. 6) over the mandrel and throughthe die.

In the foregoing figures, it is to be noted that each of the mandrels isso arranged that the bar is a tensionsustaining member which, duringextrusion, sustains essentially all of the tensile force resulting fromfriction of the extrusion metal moving forwardly over the casing. Also,in each of the illustrated mandrels the casing is acompression-sustaining member which is placed in compression and is notloaded in tension when metal is being extruded over the mandrel.

For the purpose of giving those skilled in the art a betterunderstanding of the invention and a better appreciation of theadvantages of the invention, the following illustrative example isgiven:

An extrusion billet of a difficultly formable nickelchromium-iron alloyhaving a nominal composition of about 15.8% chromium, 7.2% iron, 0.04%carbon, 0.2% manganese, 0.2% silicon and balance nickel and of acylindrical configuration about 11 inches diameter and about 25 incheshigh, with an axially oriented hole about 4% inches diameter drilledtherethrough, was heated to a temperature of about 2250 F. and placed inthe extrusion chamber of a forward extrusion press. An internallywater-cooled mandrel in accordance with the invention was attached tothe tmandrel holder of the extrusion press. The mandrel casing was ofhollow cylindrical configuration about 4 inches outside diameter, about2% inches inside diameter and about 36 inches length and was made of analloy containing about 52.3% nickel, 18.6% chromium, 5.2%columbuim-plus-tantalum, 3.1% molybdenum, 0.9% titanium, 0.5% aluminum,0.3% silicon, 0.2% manganese, 0.04% carbon and balance essentially iron.The tension bar of the mandrel was made of maraging steel having acomposition containing about 18% nickel, 9% cobalt, molybdenum, 0.5%titanium, 0.1% aluminum and 0.01% carbon and in the heat treatedcondition characterized by a room temperature yield strength of about300,000 p.s.i. The mandrel was inserted into the hole in the billet.Extrusion pressure was then exerted by the press and forced the billetmetal over the mandrel and forwardly out through an extrusion die havinga circular orifice about 4 /2 inches in diameter. The billet, whichweighed about 500 to 600 pounds, was extruded in about 16 seconds toform an extruded tube over 40 feet long with an internal diameter ofabout 4 inches. When cooled, the tube was found to be of completelysatisfactory quality without detrimental defects. The mandrel wasfurther employed to make additional extruded tubes and was still in gooduseable condition after being used for at least about 200 extrusions.Normal life of conventional mandrels in such use is satisfactory foronly about 40 or less extrusions.

Further illustration of the invention is afforded by the exemplarydimensions set forth in Table II, which shows mandrel diameters, tensionbar diameters and casing wall-thicknesses (apart from clearances) foradvantageous embodiments of the invention. The ratio of the casing Wallthickness to the mandrel diameter is advantageously about 1:5 to about1:9 for obtaining good results of long life without fracture in hotextruding heat resistant nickel-chromium alloys into tube forms havinginternal diameters from about 2 inches to about 9 inches.

TABLE II Mandrel Tension bar Casing wall diameter, diameter, thickness,

inches inches inch placed, thereby enabling continued use of the tensionbar and other components. Further, for obtaining advantages of greaterscope of utility, the new mandrel is provided with interchangeablecasings of different external diameters and thus can be used forextruding a variety of different internal diameters in tubes. Suchadvantages of extended life and greater variety of use and also otheradvantages, including protection of the tension bar from heat and wear,could not be fully realized if the casing did not extend overessentially the entire length of the tension bar, e.g., at least aboutof the working length, i.e., working surface, of the bar. Accordingly,the present invention is not to be confused with mandrels havingrelatively short lengths of special materials at portions of the mandrelexterior.

The present invention is particularly applicable in the hot extrusion ofhollow metal articles made of alloys containing a major proportion,e.g., 50% or more, of nickel, iron and/or cobalt. The invention is alsoapplicable in extrusion processes generally and is useful for coldextrusion where a mandrel having a replaceable wear-resistant outercasing and a tough, high strength, tensionsustaining interior is needed.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

I claim:

1. An extrusion mandrel, for use with an extrusion press, comprising anelongated tension bar composed of maraging steel and characterized atroom temperature by a yield strength of at least about 200,000 poundsper square inch and ductility equivalent to tensile elongation of atleast 7%, an internal fiuid coolant passage for transmitting fluidcoolant to the tension bar, a ductile casing slidably disposed inunbonded contact on said tension bar and extending over essentially theentire length of the tension bar, a retainer at the forward end of thetension bar and adapted to limit forward movement of the casing andmeans for mounting the mandrel on an extrusion press.

2. An extrusion mandrel as set forth in claim '1 having a plurality ofinternal fluid coolant passages communicating with the casing.

3. An extrusion mandrel, for use with an extrusion press, comprising anelongated tension bar composed of maraging steel and characterized atroom temperature by a yield strength of at least about 200,000 poundsper square inch and ductility equivalent to tensile elongation of atleast 7%, a ductile casing slidably disposed in unbonded contact on saidtension bar and extending over essentially the entire length of thetension bar, said casing being of a hollow cylindrical configurationwith the ratio of the cylinder wall thickness of the casing to themandrel diameter being about 1:5 to about 1:9, a retainer at the forwardend of the tension bar and adapted to limit forward movement of thecasing and means for mounting the mandrel on an extrusion press.

4. An extrusion mandrel, for use with an extrusion press, comprising anelongated tension bar composed of maraging steel and characterized atroom temperature by a yield strength of at least about 200,000 poundsper square inch and ductility equivalent to tensile elongation of atleast 7%, a ductile casing slidably disposed in unbonded contact on saidtension bar and extending over essentially the entire length of thetension bar, said casing being composed of a ductile, metallurgicallystable, heat resistant alloy containing at least about 50% of metal fromthe group consisting of nickel, cobalt and iron and characterized attemperatures from room temperature up to about 1000 F. by ductility atleast equal to about tensile elongation and by ultimate tensile strengthof at least about 120,000 pounds per square inch, a retainer at theforward end of the tension bar and adapted to limit forward movement ofthe casing and means for mounting the mandrel on an extrusion press.

5. An extrusion mandrel as set forth in claim 4 wherein the maragingsteel tension bar is characterized by a yield strength of at least about250,000 pounds per square inch and ductility equal to at least about 10%tensile elongation at room temperature.

6. An extrusion mandrel as set forth in claim 5 wherein the casing iscomposed of a precipitate-strengthened alloy containing about 50% toabout 55 nickel, about to about 21% chromium, 2% to 4% molybdenum, 4.75%to about 5.5% columbium, 0.65% toabout 1.15% titanium, 0.2% to about0.8% aluminum, up to about 0.1% carbon, up to about 0.5% silicon, up toabout 0.5% manganese with the balance essentially iron.

7. An extrusion mandrel as set forth in claim 6 having an internalcoolant passage for transmitting fluid coolant to the tension bar.

8. An extrusion mandreL'for use in forward extrusion of a metalworkpiece to form a hollow metal product, comprising an elongated casinghaving a longitudinally extending exterior surface forming essentiallythe entire portion of the mandrel surface that contacts the workpieceduring extrusion and also having a passage extending longitudinallythrough the full length of the casing;

an elongated tension bar closely fitted in unbonded contact with aninterior wall of said passage in the casing and extending longitudinallythe full length of the casing; said tension bar being composed ofmaraging steel and having a maraged microstructure characterized at roomtemperature by a yield strength of at least about 200,000 pounds persquare inch and ductility equivalent to tensile elongation of at least7%;

retaining means secured to the tension bar and limiting forward movementof the front end of the casing; and

mounting means for securing the rearward end of the tension bar to anextrusion press; said casing, tension bar, retaining means and mountingmeans being so disposed that during forward extrusion of a work pieceover the mandrel the tension bar sustains essentially all the tensileload exerted on the mandrel by friction of the forward moving workpieceand the casing is under longitudinal compression.

9. An extrusion mandrel as set forth in claim 8 having an internal fluidcoolant passage for transmitting fluid coolant to the tension bar.

10. An extrusion mandrel as set forth in claim 8 wherein the tension barhas a maraged steel microstructure characterized by at least about 10%tensile elongation at room temperature.

11. An extrusion mandrel as set forth in claim 8 wherein the tension barhas a maraged steel microstructure characterized by a yield strength ofat least about 250,000 pounds per square inch at room temperature.

12. An extrusion mandrel as set forth in claim 8 wherein the casing iscomposed of a ductile, metallurgically stable, heat resistant alloyconsisting essentially of about 12% to about 21% chromium, up to about28% iron, up to about 34% cobalt, up to about 3% titanium, up to about6% of metal from the group consisting of columbium and tantalum, up toabout 10% molybdenum, up to about 1% silicon, up to about 2% manganese,up to about 0.2% carbon with the balance essentially nickel in an amountnot less than 45% of the alloy and is characterized at temperatures fromroom temperature up to about 1000 F. by ductility at least equal toabout 10% tensile elongation and by ultimate tensile strength of atleast about 120,000 pounds per square inch.

13. An extrusion mandrel as set forth in claim 8 wherein the tension barhas a maraged steel microstructure characterized at room temperature bya yield strength of at least about 250,000 pounds per square inch and atensile elongation of at least about 10% and wherein the casing has aprecipitate-hardened microstructure characterized at temperatures fromroom temperature up to about 1000 F. by an ultimate tensile strength ofat least about 120,000 pounds per square inch and a tensile elongationof at least about 10%.

14. An extrusion mandrel as set forth in claim 13 wherein the casing iscomposed of an alloy consisting essentially of about 50% to about 55%nickel, about 15% to about 21% chromium, 2% to 4% molybdenum, 4.75 toabout 5.5% columbium, 0.65 to about 1.15 titanium, 0.2% to about 0.8%aluminum, up to about 0.1% carbon, up to about 0.5% silicon, up to about0.5 manganese with the balance essentially iron.

15. An extrusion mandrel as set forth in claim 14 having an internalfluid coolant passage for transmitting fluid coolant to the tension bar.

References Cited UNITED STATES PATENTS 2,184,048 12/ 1939 Krause 72--2732,809,750 10/ 1957 Arenz 72-264 1,989,948 2/1935 Singer 72-264 3,093,5196/1963 Decker et al. 14831 3,093,518 6/1963 Bieber 148-31 FOREIGNPATENTS 1,445,277 8 1965 France. 1,043,355 1/ 1964 Great Britain.

CHARLES W. LANHAM, Primary Examiner A. L. HAVIS, Assistant Examiner US.Cl. X.R. 72-253, 273

